High performance heat exchange assembly

ABSTRACT

Heat sinks are provided that achieve very high convective heat transfer surface per unit volume. These heat sinks comprise a spreader plate, at least three fins and at least one porous reticulated foam block that fills the space between the fins.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/189,133, filed Mar. 14, 2000.

TECHNICAL FIELD

The present invention is directed to heat sinks primarily for use indissipating waste heat generated by electrical and/or electroniccomponents and assemblies. These heat sinks include a heat spreaderplate and an assembly of heat conducting fins and reticulated foamstructures that are bonded together. Electronic components are connectedto one surface of the spreader plate with the assembly of fins and foamconnected to another surface of the spreader plate in contact with acooling fluid.

BACKGROUND OF THE INVENTION

High power electrical and electronic components continue to have anincreasing demand for higher power dissipation within a relativelyconfined space. In order to provide for such higher power dissipationrequirements while remaining suitably compact, several levels of thermalmanagement are usually required at the device, sub-assembly andcomponent level.

At the component level, various types of heat exchangers and heat sinkshave been used that apply natural or forced convection or other coolingmethods. A typical heat sink for electrical or electronic components isdepicted in FIG. 1. As shown, the heat sink includes a heat spreaderplate 10 to which metal fins 12 are attached. An electronic component isattached to face 14 of spreader plate 10 and a cooling fluid 16, such asair or water, is passed across fins 12 to dissipate the heat generatedby the electronic component. For a given power level to be dissipated,the spreader plate size (i.e., area) and the fin length along the lengthof the cooling flow path can be calculated using known methods. Finspacing and fin height are usually determined by known methods such asnumerical modeling.

In demanding applications, the method of heat exchange is usually forcedconvection to the cooling fluid. In such systems, heat exchange can beimproved by increasing the fin surface area exposed to the coolingfluid. This is accomplished by increasing the number of the fins perunit volume. However, there are limitations to achievable fin densitiesbased upon manufacturing constraints and cooling fluid flowrequirements.

Reticulated foams are also known in the art for their ability to conductheat such as the metal foams disclosed in U.S. Pat. Nos. 3,616,841 and3,946,039 to Walz, and the ceramic foams disclosed in U.S. Pat. No.4,808,558 to Park et al. Metal foams have been sold under the trade nameDUOCEL available from Energy Research and Generation, Inc., Oakland,Calif.

Until recently, metal and ceramic reticulated foams have not beenadapted for use in heat sinks for dissipating waste heat from electroniccomponents. However, these structures, especially when comprised ofmetal, make excellent heat exchangers because of their conductivity andtheir extremely high surface area to volume ratio. While earlier porousheat exchangers had up to 100 open cells per square inch, reticulatedfoam has up to 15,625 open cells per square inch. Reticulated foam isfar more porous and has far more surface area per unit volume (1600square feet/cubic foot) than heat exchangers having other structures.The pressure drop of fluids flowing through reticulated foam is alsorelatively low so that movement of a cooling fluid through the foam ispractical.

Studies by Bastawros have now shown the efficacy of metallic foams inforced convection heat removal for cooling of electronics. See,Bastawros, A.-F., 1998, Effectiveness of Open-Cell Metallic Foams forHigh Power Electronic Cooling, ASME Conf. Proc. HTD-361-3/PID-3,211-217, and Bastawros, A.-F., Evans, A. G. and Stone, H. A., 1998,Evaluation of Cellular Metal Heat Transfer Media, Harvard Universityreport MECH 325, Cambridge, Mass. Bastawros demonstrated that the use ofmetallic foam improved heat removal rate with a moderate increase in thepressure drop. Bastawros' results were based on thermal and hydraulicmeasurements (on an open cell aluminum alloy foam having a pore size of30 pores per inch) used in conjunction with a model based upon a bank ofcylinders in cross-flow to understand the effect of various foammorphologies. The model prediction was extrapolated to examine thetrade-off between heat removal and pressure drop. The measurementsshowed that a high performance cellular aluminum heat sink (i.e.,aluminum foam) removed 2-3 times the usual heat flux removed by apin-fin array with only a moderate increase in pressure drop.

SUMMARY OF THE INVENTION

A range of new heat sinks for electrical and electronic components isherein presented that provides for space-efficient heat exchange withlow thermal resistance. These heat sinks are capable of removing theincreased waste heat flux generated by today's higher power electronicsystems.

In general, heat sinks of the present invention comprise a spreaderplate, at least three fins and porous reticulated foam block that fillthe space between the fins. All materials are made from a heatconducting material. The fins and foam block form an assembly that isconnected to one surface of the spreader plate. Electronic components tobe cooled are preferably connected to an opposing surface of thespreader plate, but may be connected to any surface of the spreaderplate suited for heat transfer.

The present invention further defines the preferred dimensionalrelationships for establishing the optimum fin height for the heat sinksprovided herein. Due to the radial design of the present heat sinks, finspacing is determined by the number of fins selected. Devices producedherein find particular use in cooling microelectronic components such asmicroprocessors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a typical heat sink of the prior art.

FIGS. 2a and 2 b show the front and top views of the improved heat sinkof the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

By the present invention, it has been discovered that heat sinks thatuse a combination of solid non-porous fins and highly porous reticulatedfoam can provide improved performance over known applications that useone or the other. It is fully contemplated that any combinations of finsand reticulated foam may be used in a wide variety of differentapplications to achieve improved cooling.

It has been further discovered that there are constraints on the volumeand geometry of reticulated foam beyond which the use of additional foamwill have little impact on the overall heat sink's ability to dissipatethermal power at a given flow rate (i.e., the performance). This isbecause the reticulated foam is not a fully dense material (e.g.,aluminum foam has a density of about 10% of solid aluminum). Therefore,a high convective heat transfer surface area is achieved at the expenseof reduced thermal conductivity.

Additionally, in microelectronic cooling applications such as forexample for microprocessors, practical considerations relative topackage size, air flow rate, pressure drop and noise limits can imposefurther constraints on possible configurations and dimensions.Nonetheless, using the methods of the present invention, suitable heatsinks can be produced.

Heat sinks of the present invention achieve very high convective heattransfer surface per unit volume. These heat sinks comprise a spreaderplate, at least three fins and porous reticulated foam block that fillthe space between the fins. This basic structure may be expanded to anyconfiguration comprising foam blocks in between fins that are radiallymounted onto surface of a spreader plate.

Primary heat transfer to the cooling fluid is by convection from thefoam, with the fins and spreader plate being used to conduct heat fromthe connected heat source (i.e., the electronic component) into thefoam. In air-to-air heat exchange (i.e., where air is being used as thecooling fluid), ambient air may be drawn in through the foam's openvertical side walls and exhausted through the foam's top surface, orvice versa.

A preferred embodiment of the present invention is shown in FIG. 2a andFIG. 2b. As shown, the device comprises a heat spreader plate 20, withfins 22 and reticulated foam blocks 24 filling the space in-between thefins. The fins 22 and foam blocks 24 form an assembly that is mountedonto one surface of the spreader plate 20, leaving an opposing surfacefree for contact with an electronic component to be cooled.

Referring to FIG. 2a, fins 22 are mounted so that they are substantiallyperpendicular to the spreader plate 20. Foam blocks 24 are mountedin-between fins 22 to fill the width region that defines the horizontalspace between adjacent fins. The foam blocks 24 also preferably fill theheight region that defines the vertical space between adjacent fins tothe height of the fins 22. While FIG. 2a shows that the foam blocks 24fill the height region, it is contemplated that in alternativeembodiments the foam blocks may partially fill or overfill the heightregion.

Referring to FIG. 2b, heat sinks of the present invention are configuredsuch that the fins 22 are in a substantially radial configuration andequidistant from one another.

The selection of spreader plate size and fin length along the coolingflow length, for a given power dissipation requirement, follow thosetechniques known in the art. The overall dimensions of the spreaderplate are generally fixed based on the amount of heat to be dissipatedfrom the surface of the heat source (such as a computer chip). Thespreader plate surface area should be such that, for a prescribed flowrate of the cooling fluid flowing over the spreader plate, the heat fromthe heat source is able to spread to the edges of the spreader plate.Additional considerations may also be determinative of the spreaderplate surface area such as packaging constraints.

Generally, however, spreader plate surface area is selected bymultiplying the surface area of the heat source with the areamagnification factor. The area magnification factor λ=A_(s)/A_(h)represents a ratio of the surface area of the heat source A_(h) with thesurface area of the spreader plate A_(s). Typical values of λ are in therange of 8 to 12, and are generally used in calculating spreader platesurface area for a given surface area of a heat source. From thestandpoint of heat removal efficiency, λ should be as low as possible.Highly effective heat transfer surfaces such as highly conductive finsof optimized dimensions and/or the use of heat transfer augmentationmeans such as reticulated foam provide for relatively low values of λ.For example, in the present invention, if the surface area of the heatsource is 1.5 in² and the selected area magnification factor is taken asλ=8 (for highly efficient transfer), then the surface area of thespreader plate will be 8×1.5=12 in². For a spreader plate of this area,packaging considerations could prescribe a length of the plate in theflow direction to be 4 in. Then the width of the spreader plate will be3 in.

In preferred heat sinks, the fin (and foam) height is optimizedaccording to the following formulas. Based on heat transferconsiderations, the optimum fin height (as shown in FIG. 2a), b, isdetermined using the relation, $\begin{matrix}{b = {0.6498\sqrt{\frac{k_{f}\delta_{f}}{h}}}} & (1)\end{matrix}$

where,

k_(f) is the thermal conductivity of the fin material, Btu/ft s ° F.

δ_(f) is the fin thickness, ft

h is the convective heat transfer coefficient for the foam-filled spacebounded by the fins and the spreader plate, Btu/ft² s ° F., and where,$\begin{matrix}{h\quad = \quad {{1.2704\quad\left\lbrack \quad \frac{n^{0.50}}{\left( {1\quad - \quad \varphi} \right)^{0.25}} \right\rbrack}\quad \left( \frac{\rho^{0\quad {.50}}\quad k^{0.\quad 63}\quad c_{p}^{0\quad {.37}}}{\mu^{0\quad {.13}}} \right)\quad u_{m}^{0.50}\quad \Psi}} & (2)\end{matrix}$

where,

n is the linear density of the foam material, pores per ft

Φ is the foam porosity expressed as a fraction

ρ is the density of the flowing fluid, lb_(m)/ft³

k is the thermal conductivity of the flowing fluid, Btu/ft s ° F.

c_(p) is the isobaric specific heat of the flowing fluid, Btu/lb_(m) °F.

μ is the dynamic viscosity of the flowing fluid, lb_(m)/ft s

u_(m) is the mean velocity of the flowing fluid, ft/s

ψ is the dimensionless flow efficiency factor

The flow efficiency factor ψ accounts for the secondary flows andcoexisting laminar and turbulent flows in the corner region formed bythe fins in the center of the spreader plate. For the parallel finconfiguration with no corner effect ψ=1, while for the radial finconfiguration of the present invention with the corner effect, ψ<1. Forheat sinks of the present invention, the preferred value for ψ is about0.56.

To maximize thermal conduction from the heat source through the spreaderplate into the fins and foam, the fins and foam are bonded to oneanother and the spreader plate. While thermal bonding such as brazing ispreferred, any suitable means may be employed including, for example,using a conductive epoxy to form an adhesive bond. In preferred heatsinks of the present invention, the fins, the foam blocks and thespreader plate are assembled and then preferably furnace-brazed to bondthe foam to the fins and the spreader plate.

The spreader plate and fins are solid and made from thermal conductingmaterials known in the art. The reticulated foam is an open cell mediaand also made from known thermal conducting materials. Preferred thermalconducting materials include aluminum, copper, graphite andaluminum-nitride ceramics. The spreader plate, fins and the reticulatedfoam may be selected from the same or different materials. In apreferred embodiment, the selected thermal conducting material for thespreader plate, fins and the reticulated foam is aluminum.

The reticulated foam structure is available from commercial sources ormay be made by methods known in the art. Suitable processes for makingmetal foams are disclosed in U.S. Pat. Nos. 3,616,841 and 3,946,039 toWalz, and processes for making ceramic foams are disclosed in U.S. Pat.No. 4,808,558, the teachings of which are incorporated herein byreference. Reticulated foam metal can be formed by the manufacturer tohave many shapes, densities and cell sizes. Foam blocks as used hereinmay be obtained from such manufacturers or cut from larger pieces.Aluminum foams suitable for use herein are available under the tradenameDUOCEL from Energy Research and Generation, Inc., Oakland, Calif.

The following examples are provided to illustrate heat sinks of thepresent invention designed for microelectronic cooling applicationsusing the relationships set forth above and based upon a powerdissipation requirement of up to about 200 watts.

EXAMPLE 1

In a heat sink of the present invention, aluminum fins are selected, tobe deployed in a radial fin pattern, having a thickness of δ_(f)=0.125inch (0.0104 ft) with thermal conductivity k_(f)=133 Btu/ft hr ° F.(0.0369 Btu/ft s ° F.). A commercially available open cell aluminum foamwith linear density n=20 pores per inch (240 pores/ft) and porosityΦ=0.90 is also selected. The cooling medium is the ambient air flowingwith a mean velocity u_(m)=10 ft/s. The value of the flow efficiencyfactor ψ is 0.56. The transport properties of the ambient air are asfollows.

Density ρ=0.0749 lb_(m)/ft³

Thermal conductivity k=0.0000041 Btu/ft s ° F.

Isobaric specific heat c_(p)=0.2410 Btu/lb_(m) ° F.

Dynamic viscosity μ=0.0000123 lb_(m)/ft s

To determine optimum fin height, b, the convective heat transfercoefficient, h, is first determined using Equation (2), above,providing, h=0.0175 Btu/ft² s ° F. Next, introducing this value of hinto Equation (1), we obtain the optimum fin height b=0.0964 ft. (1.1563inch).

EXAMPLE 2

This example is the same as Example 1 except that the fin material hasbeen changed from aluminum to copper. The copper fins have a thicknessδ_(f)=0.125 inch (0.0104 ft) with thermal conductivity k_(f)=226 Btu/fthr ° F. (0.0628 Btu/ft s ° F.). The reticulated foam is a commerciallyavailable open cell foam having a linear density n=20 pores per inch(240 pores/ft) and porosity Φ=0.90. The cooling medium is the ambientair flowing with a mean velocity u_(m)=10 ft/s. The value of the flowefficiency factor ψ is 0.56. The transport properties of the ambient airare as follows.

Density ρ=0.0749 lb_(m)/ft³

Thermal conductivity k=0.0000041 Btu/ft s ° F.

Isobaric specific heat c_(p)=0.2410 Btu/lb_(m) ° F.

Dynamic viscosity μ=0.0000123 lb_(m)/ft s

As in Example 1, using Equation (2), the convective heat transfercoefficient h=0.0175 Btu/ft²s ° F. Then, using Equation (1), we obtainthe optimum fin height b=0.1256 ft (1.5073 inch). This optimal height ofthe copper fin is 30% higher than that for the aluminum fin indicatingthat for the same fin thickness the copper fin has higher heatdissipation potential than aluminum fin. This can be attributed to thehigher thermal conductivity of copper.

While the preferred embodiment of the present invention has beendescribed so as to enable one skilled in the art to practice the heatsinks disclosed, it is to be understood that variations andmodifications may be employed without departing from the concept andintent of the present invention as defined by the following claims. Thepreceding description and examples are intended to by exemplary andshould not be read to limit the scope of the invention. The scope of theinvention should be determined only by reference to the followingclaims.

What is claimed is:
 1. A heat sink for electrical or electroniccomponents comprising: a heat spreader plate to which the components tobe cooled are connected; at least three heat conducting fins positionedadjacent to one another in a substantially radial configuration andwhich are connected substantially perpendicular to said heat spreaderplate; highly porous heat conducting reticulated foam block that fillsthe space between adjacent fins; and wherein: the fin height, b, isdetermined by the relationship,$b = {0.6498\sqrt{\frac{k_{f}\delta_{f}}{h}}}$

where, lc_(f) is the thermal conductivity of the selected fin material,Btu/ft s ° F. δ_(f) is the fin thickness, ft h is the convective heattransfer coefficient for the foam-filled space bounded by the fins andthe spreader plate, Btu/ft² s ° F., and where h is given by the formula,$h = {{1.2704\quad\left\lbrack \frac{n^{0.50}}{\left( {1 - \varphi} \right)^{0.25}} \right\rbrack}\quad \left( \frac{\rho^{0.50}k^{0\quad {.63}}c_{p}^{0\quad {.37}}}{\mu^{0.13}} \right)u_{m}^{0.50}\Psi}$

where, n is the linear density of the foam material, pores per ft ø isthe foam porosity expressed as a fraction ρ is the density of theflowing fluid, lb_(m)/ft³ k is the thermal conductivity of the flowingfluid, Btu/ft s ° F. c_(p) is the isobaric specific heat of the flowingfluid, Btu/lb_(m) ° F. μ is the dynamic viscosity of the flowing fluid,lb_(m)/ft s u_(m) is the mean velocity of the flowing fluid, ft/s Ψ is<1.
 2. A heat sink of claim 1 wherein said fins and said foam blocks areconnected to one surface of said heat spreader plate.
 3. A heat sink ofclaim 1 wherein Ψ is 0.56.
 4. A heat sink of claim 1 wherein said heatspreader plate, said fins and said heat conducting foam are made fromthe same or different thermal conducting materials.
 5. A heat sink ofclaim 1 wherein said heat spreader plate, said fins and said heatconducting foam are made from aluminum, copper, graphite oraluminum-nitride ceramic.
 6. A heat sink of claim 1 wherein said heatspreader plate, said fins and said heat conducting foam are made fromaluminum.
 7. A method of making a heat sink comprising a heat spreaderplate, at least three fins and reticulated foam block that fills thespace in-between the fins comprising, selecting said heat spreaderplate, said fins and said foam block; wherein the fin height, b, isdetermined by the relationship,$b = {0.6498\sqrt{\frac{k_{f}\delta_{f}}{h}}}$

where, k_(f) is the thermal conductivity of the selected fin material,Btu/ft s ° F. 67 _(f) is the fin thickness, ft h is the convective heattransfer coefficient for the foam-filled space bounded by the fins andthe spreader plate, Btu/ft² s ° F., and where h is given by the formula,$h = {{1.2704\quad\left\lbrack \frac{n^{0.50}}{\left( {1 - \varphi} \right)^{0.25}} \right\rbrack}\quad \left( \frac{\rho^{0.50}k^{0\quad {.63}}c_{p}^{0\quad {.37}}}{\mu^{0.13}} \right)u_{m}^{0.50}\Psi}$

where, n is the linear density of the foam material, pores per ft ø isthe foam porosity expressed as a fraction ρ is the density of theflowing fluid, lb_(m)/ft³ k is the thermal conductivity of the flowingfluid, Btu/ft s ° F. c_(p) is the isobaric specific heat of the flowingfluid, Btu/lb_(m) ° F. μ is the dynamic viscosity of the flowing fluid,lb_(m)/ft s u_(m) is the mean velocity of the flowing fluid, ft/s Ψ is<1; assembling said fins and said foam block onto said spreader plate sothat said fins are adjacent to one another in a substantially radialconfiguration and substantially perpendicular to said spreader plate andsaid foam block fill the space in between adjacent fins; and bonding theassembly of said fins and said foam block to said spreader plate.
 8. Amethod of claim 7 wherein the assembly of said fins and said foam blockare connected to one surface of said heat spreader plate.
 9. A method ofclaim 7 wherein said bonding is accomplished using a thermallyconductive adhesive or furnace brazing.
 10. A method of claim 7 whereinΨ is 0.56.
 11. A method of claim 7 wherein said heat spreader plate,said fins and said heat conducting foam are made from the same ordifferent thermal conducting materials.
 12. A method of claim 7 whereinsaid heat spreader plate, said fins and said heat conducting foam aremade from aluminum, copper, graphite or aluminum-nitride ceramic.
 13. Amethod of claim 7 wherein said heat spreader plate, said fins and saidheat conducting foam are made from aluminum.
 14. A method of coolingelectronic components by attaching the electronic components to onesurface of a heat sink and passing a cooling fluid over the opposingsurface of the heat sink, wherein said heat sink comprises, a heatspreader plate, at least three heat conducting fins that are adjacent toone another and positioned in a substantially radial configuration andwhich are connected substantially perpendicular to said heat spreaderplate, and highly porous heat conducting reticulated foam block thatfills the space between adjacent fins, wherein the height of said fins,b, and is determined by the relationship,$b = {0.6498\sqrt{\frac{k_{f}\delta_{f}}{h}}}$

where, k_(f) is the thermal conductivity of the selected fin material,Btu/ft s ° F. δ_(f) is the fin thickness, ft h is the convective heattransfer coefficient for the foam-filled space bounded by the fins andthe spreader plate, Btu/ft² s ° F., and where h is given by the formula,$h = {{1.2704\quad\left\lbrack \frac{n^{0.50}}{\left( {1 - \varphi} \right)^{0.25}} \right\rbrack}\quad \left( \frac{\rho^{0.50}k^{0\quad {.63}}c_{p}^{0\quad {.37}}}{\mu^{0.13}} \right)u_{m}^{0.50}\Psi}$

where, n is the linear density of the foam material, pores per ft ø isthe foam porosity expressed as a fraction ρ is the density of theflowing fluid, lb_(m)/ft³ k is the thermal conductivity of the flowingfluid, Btu/ft s ° F. c_(p) is the isobaric specific heat of the flowingfluid, Btu/lb_(m) ° F. μ is the dynamic viscosity of the flowing fluid,lb_(m)/ft s u_(m) is the mean velocity of the flowing fluid, ft/s Ψ is0.56.
 15. A method of claim 14 wherein the electronic component is amicroprocessor, the cooling fluid is air and the heat sink is made fromaluminum materials.
 16. A method of claim 15 wherein the air is pushedthrough said foam blocks.